Plasma arc cutting is a process in which an electric arc is used to cut a metallic workpiece. Generally, plasma arc cutting uses an electric arc between an electrode and the metal to be cut. The arc creates a plasma that cuts the metallic workpiece. Plasma cutters are often used in applications such as building maintenance (structural or HVAC) where they are carried from job to job, and thus are preferably highly portable.
A typical prior art plasma arc cutter includes a power supply, and/or an air supply, and/or torch. A torch or plasma torch is used to create and maintain the arc and plasma jet that perform the cutting. A plasma cutting power supply receives an input voltage (from a power line or generator) and provides an output voltage to a pair of output terminals, one of which is the electrode and the other of which is connected to the workpiece. There are numerous types of known plasma arc cutting power supplies, such as magnetic power supplies, inverter power supplies, phase control power supplies, and choppers or secondary switchers. The Miller Spectrum.TM. 300 plasma power supply is an example of a prior art chopper type power supply. Plasma cutting power supply, as used herein, includes one or more of the input power lines, transformers, convertors/inverters etc, output power lines, and controller, that cooperate to provide power to the arc.
An air supply is used with most plasma cutters to help start the arc, control the torch, and cool the torch. U.S. Pat. No. 4,791,268, to Sanders, et al., describes a plasma torch controlled with air. A movable electrode is the cathode and a fixed nozzle is the anode. A pilot current is provided to the cathode and anode, which are forced into contact by a spring. After electrical current begins to flow from the electrode to the nozzle, gas or air supplied to the torch counteracts the spring force and moves the electrode away from the nozzle. This breaks the electrical contact between the electrode and the nozzle and creates the pilot arc. Also, as the electrode moves away from the nozzle, it opens a nozzle orifice (connected to the air supply), and a plasma jet is created.
The plasma jet causes the arc to transfer (at least in part) to the work piece, thus initiating cutting. Electronics in the power source sense when the arc has transferred and then supply a greater magnitude main cutting current after the transfer has occurred. Also, the torch tip is disconnected (electrically) interrupting the pilot current path. Thus, the current is used to cut the workpiece, and follows a path including the positive terminal, the workpiece, and the electrode.
An example of such a system is described in U.S. Pat. No. 5,660,745, entitled Method And Apparatus For A Contact Start Plasma Cutting Process, issued Sep. 28, 1999, to Naor, and assigned to the assignee of the present invention, and hereby incorporated by reference. As described therein, air flow is provided after the cutting has ceased (post flow) for post-arc cooling is desirable.
The prior art describes several ways to provide air for the plasma process. A tank of air has been provided. This may provide a supply of air, but it limits portability. Another system uses a shop-source of air (such as a central compressor). This obviously limits portability by requiring a nearby "shop-air" outlet.
Another prior art system uses an air compressor separate from the plasma cutter. Such a compressor may simply be an off-the-shelf compressor, and uses a separate power source, is controlled separately, and is in a separate housing. Such a compressor is not an integrated compressor, and not highly portable. Also, such prior art does not typically provide air only when needed, nor does it provide air always when needed, for example for post flow. An air compressor is integrated into the plasma cutter when the compressor shares at least one of a common housing, a common power source, or a common control signal or circuit with the power supply. The separate compressor also limits portability, and is not controlled specifically with plasma cutting in mind.
Other prior art systems include an air compressor integrated with the plasma cutter, but the compressor is either unregulated, or the air pressure is regulated using a mechanical valve.
A compressor is regulated when the motor is controlled in response to feedback such as motor current, voltage, functions of current and voltage, motor rpm, output pressure, air flow, etc. An unregulated compressor is often oversized to be sure to provide adequate air, and is not controlled with the plasma cutter, so that it runs at times when no air is needed. This may be expensive and inefficient. Also, a larger than necessary air supply reduces portability. Examples of an unregulated integrated air supply is the Miller Spectrum.RTM. 187D, and Thermal Dynamics DragGun.TM..
Prior art also teaches regulated air pressure by pressurizing a tank with a compressor to a pressure greater than that needed. A mechanical valve is controlled to regulate the pressure down to a desired value. Thus, the pressure is regulated, but the compressor is not regulated. One such system provided that either the compressor ran (charging the air tank), or cutting could be performed, but not both, thus limiting the current draw. Such systems may be inefficient, and heavier than necessary, because the compressor is over-sized, and runs more than actually needed to build up a high enough pressure to be regulated down to the desired pressure. Examples of this type of prior art include a Hypertherm AirPak.TM..
Accordingly, a plasma cutter with an integrated air compressor is desired. Preferably, the compressor will be regulated so that it can be appropriately sized and controlled. Also, preferably, the air compressor is integrated with the cutter by having one or more of a common housing, a common control circuit or signal, and a common power supply. Preferably, such a plasma cutter will have an air compressor that is turned on only when needed, and controlled with the cutter.